This should be a mostly non breaking change for hot reload workflows that improves performance, but if you see anything odd with hot reload behavior on master please file a bug. This feature can also be disabled without any changes to the SDK with flutter config --no-single-widget-reload-optimization 

Delete the destination directory for the web sdk before copying the new contents. This might fix an error where the OS refuses to overwrite the files on dev

* migrate some cupertino files to nullsafety

* address review comments

Improve our ability to measure the performance of hot restart. Also flush all UI thread tasks before calling the hot restart "complete" in benchmark mode.

If this is successful at stabilizing the benchmarks, this should be enabled for "normal" hot restart so that we can accurately track execution time.

Based on feedback from various desktop developers, and to be consistent between the defaults and the sample code, this PR switches the default for visual density in desktop themes to be compact instead of standard.

It also removes the same setting from the sample code generated by "flutter create" because it is no longer needed now that it is the default.

Updates all null safe dependencies to versions that allow 2.10 stable and 2.11 dev releases.

Also updates flutter_goldens and flutter_goldens_client to allow 2.11 dev.

The throttle duration could delay past the point where the destination sink was closed. Check if it is closed before adding an event. Fixes a crash on dev: StateError: Bad State: Stream is already closed.

If a file scheme and one or more roots is provided, fall back to this mapping before the direct file path if the file path cannot be turned into a package URI.

Use URI representation so that the transformation is resilient to the org-dartlang-app scheme used by the web builds.

Fixes #66095
Fixes #66404

* migrate some cupertino files to nullsafety

* address review comments

* address review comments

Wrapping the spacing SizedBox in Flexible.

This reverts commit af6ba867.

[flutter_tools] only lock if an upgrade/download will be performed (linux/macos) and output building messages to stderr (#65422)

Currently an invocation of flutter/dart will always attempt to acquire a lock. This can pose problems for tools that attempt to run multiple dart/flutter instances.

Instead update the lock logic (on Linux/macOS) so that we only attempt to acquire it if an update/snapshot needs to be performed. To avoid repeatedly performing downloads/snapshots if multiple flutter/dart invocations are fired off concurrently when an update needs to be performed, do a second check of the download/snapshot condition after the lock is released.

Additionally, moves all of the building/debug output to stderr on both the bash and batch scripts. This allows machine mode consumption of the tool to ignore needing to parse/handle the rebuild messages.

If a file scheme and one or more roots is provided, fall back to this mapping before the direct file path if the file path cannot be turned into a package URI.

Fixes #66095

Revert "Revert "Ensure VmService instance is disposed after failed direct connection attempt (#66123)" (#66164)" (#66377)

This reverts commit 3e6863ab.